[Haskell-cafe] classes with types which are wrapped in
Miguel Mitrofanov
miguelimo38 at yandex.ru
Fri Jan 22 11:07:12 EST 2010
Wrap it in a newtype. That's the only way I know.
Andrew U. Frank wrote:
> i encounter often a problem when i have a class with some operations (say
> class X with push) applied to a type A b. I then wrap A in a type A_sup, with
> some more type parameters and i cannot write a instance of class A_sup because
> i have a kind mismatch. any suggestions? (reordering of the type parameters of
> A_sup is not a solution, because another class operates on this parameter)
>
> here a simplistic case (i know that A could be reduced to [], my real cases
> are more complicated).
>
> data A b = A b [b]
>
> data Asup x ab y = Asup x ab y
>
> class X a b where
> push :: b -> a b -> a b
>
> instance X A Int where
> push b' (A b bs) = A b' (b:bs)
>
> instance X Asup Char Int Float where
> push b' (Asup a b c) = Asup a (push b' b) c
>
> -- this does not compile because the number of type arguments for X is wrong.
>
> if i try with a type
>
> type A_2 b = Asup Char (A b) Float
>
> instance X A_2 Int where
> push b' (Asup a b c) = Asup a (push b' b) c
>
> (and --TypeSynonymInstances) i get:
>
> Type synonym `A_2' should have 1 argument, but has been given 0
> In the instance declaration for `X A_2 Int'
>
> what is the solution? thank you!
> andrew
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